Monoethylene glycol (MEG) is commonly used in the oil and gas industry to prevent the formation of gas hydrates in pipelines. The MEG is injected at the wellhead and readily mixes with the produced water to form a mixture referred to as rich MEG.
The oil and rich MEG are separated in the final production separator. However, depending upon the properties of the oil, the hydrocarbon content of the rich MEG may be as high as 1,000 parts per million (ppm). When the oil properties are in the range of a light condensate, the hydrocarbon content may range from 100 ppm to 200 ppm, but even these concentrations may be excessive for optimal operation of the MEG reclamation process.
Increasing the removal of hydrocarbons from the rich MEG will improve the operation of the MEG reclamation process. In addition, hydrocarbon removal eliminates or significantly reduces potential difficulties during the reclamation process. As an example, the rich MEG feed stream must be heated in a heat exchanger before it enters the flash separator. However, the heating process may convert some of the hydrocarbons to a black solid charcoal coke-like material that remains in suspension inside the flash separator. As more of this material forms, it increases the rich MEG's tendency to form a stable foam, reduces the settling of salt crystals, increases the abrasive nature of the rich MEG so that pump seal failure and pipe erosion are more prevalent, and discolors the salt produced in the reclamation process making it unsuitable for marine discharge. In offshore operations, excessive hydrocarbon levels in the rich MEG feed stream can also cause plugging of the downstream lean MEG injection nozzles if the hydrocarbons form a solid compound under line or injection conditions. As another example, hydrocarbons carried with the rich MEG feed stream into the distillation column of the MEG reclamation process must either exit with the distilled water or with the lean MEG. In either case, the hydrocarbons may need to be removed to meet water discharge or product purity specifications. As a final example, hydrocarbons in the rich MEG feed stream may coat the ion exchange resin, reducing the efficiency of any process to remove divalent cations.
Hydrocarbon removal beyond the final production separator is currently limited. Activated charcoal filters are generally used for such removal and are capable of reducing the hydrocarbon levels to the range of approximately 25 to 50 ppm. However, because activated charcoal filters are not very efficient, they are heavy and require large amounts of space and volume. This additional space and weight is very expensive, particularly for offshore operations. In addition, the charcoal filter material must be replaced whenever it is fully adsorbed with hydrocarbons. The spent material may be disposed of as waste or regenerated onshore. Periodically replacing the filter material and properly handling the spent material further increase the costs associated with activated charcoal filters.
In addition to hydrocarbons, the rich MEG feed stream may be loaded with dissolved salt ions from the produced water. Although sodium chloride is commonly the most concentrated salt in the produced water, the feed stream may also contain dissolved divalent salts of magnesium, calcium, strontium, and barium. If these ions are not removed before the MEG and water are separated, they will precipitate and accumulate in the process equipment, eventually leading to failure of the reclamation process.
In the current process for separating divalent cations from the rich MEG feed stream, the cations react with additional carbonate or hydroxide to form insoluble salt crystals, which are then removed from the feed stream. This process generally requires the addition of caustic and acid to completely remove the divalent cations and to neutralize the feed stream before it enters the MEG reclamation process.
The time and temperature of the current separation process must be strictly controlled. In addition, the process requires large and expensive equipment, as well as additional chemicals that are not inherently available as part of the MEG reclamation process. These chemicals must be obtained from outside sources which can be very expensive, particularly when delivered to offshore platforms in remote parts of the world. The chemicals may also be a safety concern, require specialized handling and storage, and increase training, reporting, and recordkeeping requirements. The current separation process also produces a carbonate salt in the form of a solid or slurry material that is generally insoluble and requires disposal as a waste. Proper disposal of this material can be expensive, time-consuming, and labor-intensive. Disposal is even more difficult in offshore applications where temporary storage space and transportation to an approved disposal site are not readily available.
A need exists for systems and processes to remove hydrocarbons and divalent cations from rich MEG feed streams in order to improve the efficiency and eliminate problems during the MEG reclamation process. A need also exists for systems and processes that are less expensive, easier to operate, do not require large amounts of space or additional chemicals, can be regenerated without removing and processing the adsorbent material, and facilitate the disposal of process waste streams.